Loosen the constraints of iterators

[inmaldrerah]

The current iterator constraints are:

• Have Self type
• Have Item type
• Have next method takes *Self and returns ?Item.

But I think the Self and Item types should not be enforced, as we can find them from the signature of the next method. Also, many iterators in the standard library do not have these two associated types, or have Self named in another way, e.g. std.bit_set.IntegerBitSet.Iterator.

Instead, we can define an iterator to be a type T that has a next function, taking one parameter of type *T and returns any value of optional type.

We can check this constraint with the domain and codomain functions in src/compat.zig.